The present invention relates to a method and a correspondingly designed device for transmitting data according to a hybrid ARQ method in a communication system; in particular, a mobile radio system.
The use of so-called packet access methods or packet-oriented data connections is often recommended in connection with mobile radio systems, since the message types produced often have a very high burst factor with the result that only short periods of activity exist, interrupted by long rest phases. Packet-oriented data connections may, in this case, considerably increase efficiency compared to other data transmission methods in which a continuous data stream is present, since, in data transmission methods with a continuous data stream, once a resource has been allocated, such as a carrier frequency or a time slot, it remains allocated during the entire communication relationship. That is, a resource remains occupied even if there are momentarily no data transmissions taking place, wherein the resource is not available for other network users. The result is that the narrow frequency range available for mobile radio systems is not used to best effect.
Future mobile radio systems, such as those that comply with the mobile radio standard UMTS (Universal Mobile Telecommunications System), will offer a multitude of different services whereby multimedia applications will become increasingly prevalent alongside pure voice transmission. The diversity of services associated with this, with different transmission rates, requires a highly flexible access protocol on the air interface of future mobile radio systems. Packet-oriented data transmission systems have proved to be highly suitable in this context.
In connection with UMTS mobile radio systems, a so-called ARQ (Automatic Repeat Request) method has been proposed in packet-oriented data connections. In this method, the data packets transferred from a transmitter to a receiver are checked for quality at the receiving end following decoding. If a data packet is errored on receipt, the receiver requests retransmission of the data packet by the transmitter; i.e., a repeat data packet which is identical or partially identical to the one previously sent and which was errored on receipt is sent from the transmitter to the receiver, (the terms full or partial repeat are used to indicate whether the quantity of data contained in the repeat data packet is less than or equal to that of the original data packet). With regard to the ARQ method proposed for the UMTS mobile radio standard, the intention is for both data and so-called header information to be transmitted in a data packet, whereby the header information also contains information for error checking, such as CRC (Cyclic Redundancy Check) bits, and also can be coded for error correction (known as FEC, Forward Error Correction).
In accordance with the current proposals for the UMTS specification, it is planned to introduce a so-called hybrid ARQ method which is also known as the Incremental Redundancy ARQ method. The main difference compared to the conventional ARQ method described above is that a coding process is carried out in the receiver using several data packets with the same information source but different channel coding. In other words, several data packets with the same information source are decoded and evaluated until an originally transmitted data packet is deemed to have been decoded as error-free in the receiver. In contrast to the previously described ARQ method, in the hybrid ARQ method the repeat data packets (which are also called “retransmission”) are not normally identical to the data packet originally transmitted.
FIG. 3 shows the signal processing for the hybrid ARQ method under discussion in accordance with the current status of UMTS standardization.
As shown in FIG. 3, the generation of the transmission signal is subdivided into the processing of the actual data and the processing of the header information. On the header side, the header information created by a function block 3 is supplied to a function block 12, which ensures that all headers of all data packets that are to be transmitted in the same radio packet are linked together into a single header (header concatenation). A function block 13 inserts CRC bits for error detection into the resulting header information. The resulting bit stream is then subjected to channel coding by a function block 14 and rate matching by a function block 15. An interleaver 16 causes the symbols and/or bits supplied to it to be arranged in a particular way and time-interleaved. The data blocks produced by the interleaver 16 are allocated by a function block 17 to the individual transmit or radio frames (known as radio frame segmentation).
A function block 4 is also provided on the data side for the insertion of CRC bits. A function block 5 is used for splitting the data supplied to a channel coder 6 such that a coding process limited to a specific number of bits can always be carried out by the channel coder 6.
Because of the channel coding carried out by the channel coder 6, the actual data to be transmitted has redundant information inserted in it with the result that several consecutively transmitted data packets have bits with the same information origin. In the hybrid ARQ method (as per so-called Types II and III), if a data packet is incorrectly received or decoded by the receiver, the transmitter does not send the same packet again to the receiver but instead transfers a data packet which has at least some bits with the same information source as the bits contained in the data packet originally transferred. The receiver, by evaluating together all data packets containing bits with the same information source, can retrieve the originally transmitted information with better quality.
In order to implement the hybrid ARQ method described above, it is necessary, if so requested by the receiver, for the data transmission signals with incremental redundancy which are received in the originally transmitted data packet and the corresponding repeat data packets to be generated from the same information source (so-called redundancy selection). For this purpose, after the channel coder 6 a corresponding function block 7 is provided, which is controlled by the function block 3 as a control entity and carries out a corresponding bit selection process after the channel coding. In this way, the bits to be transmitted in the first packet and in repeat data packets are selected by the function block 7 according to the control exercised by the function block 3.
The data produced by function block 7 is supplied to a function block 8, which adjusts the bit rate of the data stream by masking or omitting individual bits (known as puncturing) or repeating individual bits. So-called DTX (Discontinuous Transmission) bits can be inserted into the data stream by a subsequent function block 9. Also provided on the data side are function blocks 10 and 11, which perform the same functions as do function blocks 16 and 17 on the header side.
Finally, the bits produced on the data and header side are mapped or multiplexed by a function block 18 on whatever physical transfer or transmission channel is available at the time.
To ensure that the hybrid ARQ method or incremental redundancy ARQ method operates with maximum efficiency, the FEC coding of the various data repeat packets should be selected, if possible, such that the data packets typically have very little concurrence between them. This is particularly problematic in the case of UMTS because of the high flexibility of the channel coding and data matching carried out with regard to the individual transmission channels; in particular, since the channel coding cannot be optimized solely in terms of its efficiency. Other factors are also involved in this; for example, the memory requirement and the complexity of the algorithms executed, an essential role since this particularly affects the algorithms executed on the part of the mobile station.
In general, the problem with the hybrid ARQ method explained using FIG. 3 is that of how best to code an originally transmitted data packet, and possibly one or more subsequently transmitted repeat data packets, in order to achieve maximum benefit.
The present invention is, therefore, directed toward a method and a correspondingly designed device for transmitting data according to a hybrid ARQ method, that enables the principle of a hybrid ARQ method to be implemented with increased flexibility and better use of the potential coding gain without significantly increasing the complexity and the memory requirement.